Process of improving the dyeing properties of polyvinyl alcohol fibers



Marh 28, 1951 TETsURo osUGl ET AL 2,977,183

PROCESS OF IMPROVING THE DYEING PROPERTIES OF POLYVINYL ALCOHOL FIBERS Filed Deo. 19, 1952 2 Sheets-Sheet l March 28, 1961 TETsURo osUGl l-:TAL 2,977,183

RRocEss 0E IMPROVING THE DYEING PROPERTIES OF POLYVINYL ALCOHOL FIBERS 2 Sheets-Sheet 2 Filed DSC. 19, 1952 INVENTORSI BY dfn/wa Ma/Mam PROCESS OF lMPRViNG THE DYEING PROP- ERTIES OF POLYVINYL ALCOHOL FIBERS Filed Dec. 19, 1952, Ser. No. 326,892

Claims priority, application Japan Dec. 19, 1951 6 Claims. (Cl. lll-54) The invention relates to a process for improving the dyeing properties of polyvinyl alcohol fibers and rendering the same resistant to boiling water. f

Polyvinyl alcohol fibers as obtained by dry or wet spinning methods from aqueous polyvinyl alcohol solutions shrink considerably in water at room temperature Y and dissolve in hot water at 60 to 80 C.; they are, therefore, unsuited for textile purposes and have to be subjected to an after-treatment in order to increase their water resistance. Y

One of the best known methods for insolubilizing polyvinyl alcohol in hot or boiling water is by acetalization with formaldehyde. Another method whereby polyvinyl alcohol fibers are not only insolubilized but also made unshrinkable in boiling Water consists in subjecting polyvinyl alcohol fibers, prior to their acetalization, to a heat treatment at temperatures of 140 to 225 C., preferably in an unsaturated atmosphere of water or alcohol vapors, as disclosed in the copending applications for patent bv Tsukumo Tomonari and Shigeki Nomura, Serial Nos. 154,873 and 300,375, led April 8, 1950, and July 23, 1952, respectively.

By said heat treatment the softening temperature, i.e. the temperature at which the fiber shrinks of its length in water is raised to 80-*96 C., 'and the subsequent acetalization raises the softening point further to above 100 C.

When a water-sensitive polyvinyl alcohol fiber is directly acetalized after spinning without the heat treatment referred to hereinabove, the fiber shrinks more than 10 percent of itslength during the acetalization and the fibers tend to sticktogether after drying unless the acz etalization bat'n contains a large amount of. a strong coagulating agent for polyvinyl alcohol. Therefore, the conventional acetalizing solutions contain a powerful coagulating agent such as sodium or ammonium sulfate in an amount of at least percent. n

While the combination of heat treatment and subsequent 'acetalization according to the above-mentioned method accomplishes the desired aim-ofmaking polyvinyl alcohol fibers resistant to boiling water, it has the drawv 15 percent to the formaldehyde solutions used for the direct acetalization of freshly spun water sensitive polyvinyl alcohol fibers.

We have found that in acetalizing polyvinyl alcohol fibers which already have a high softening point of at least 80 C. it is not necessary to use such high concentrations of salt such as sodium or ammonium sulfate and that, on the contrary, the use of acetalizing baths having much lower concentrations of the salt than employed heretofore results in a surprising and unexpected increase of the dyestuf afiinity of the fiber.

The acetalizing bath according to our invention contains an aldehyde, or a compound giving off aldehyde of 0.5 to `l5 percent, preferably 2 to 8 percent, an inorganic 'acid' in Ia concentration of about 0.5 to 25 percent, and as critical feature an alkali metal or ammonium `salt in a concentration of less than 5 percent, all figures given by weight.

The obtained acetalization degree depends on the coml position of theacetaliz'ing bath, on the reaction temperature and time as well as on the properties of the treated polyvinyl alcohol fiber. The use of higher concentrations Vof acid or aldehyde, and also the use of higher temperatures, result in. greater degrees of acetalization. The effect of the salt concentration on the degree of acetalization is generally small, but the extent of the preceding heat-treatment exerts a certain influence on the acetalization degree insofar as the higher the softening point (in water) of the heat-treated polyvinyl alcohol liber is, the slower is the rate of acetalization. But this difference isA rather small in the vhere interesting softening point range (in water) between 80 C. and 96 C. The effect ofV the vfiber diameter is noticeable., but can be disregarded for fibers having 1-20 denier.

The preferred aldehydes are lower aliphatic aldehydes or compounds yielding such aldehydes or decomposable thereto by lacids, suchv as their polymers and amination products. Examples `are formaldehyde, paraforrnaldehyde, polyoxymethylene, hexarnethylenetetramine, iacetaldehyde, paraldehyde, and the like. As the inorganic acid, we prefer to use sulfuric acid, but other acids such as hydrochloric acid or even phosphoric or nitric acid can also be used. If sulfuric acid is replaced byy such other acid, the concentration of,.for instance, hydrochloric acid should be generally somewhat lower, and that of phosphoric acid somewhat higher, 'than the concentration of the sulfuric acid. fAs inorganic salt we employ preferably a saltfor a mixtureof salts of the acid used, in the case of sulfuric acid, for instance,

sodium sulfate and/or ammonium sulfate.Y

back of reducing considerably the-affinity of the polyvinyl i alcohol fibers to dyestuifs,- particularly to direct dyestuffs.

It has been extremely difficult to dye heat-treated and then partially acetalizedV polyvinyl alcohol fibers under normal dyeing conditionsto medium or heavyl shades; This is true of direct, basic, acid, sulfide, vat, naphthol and even acetate dyestuifs.y p f j j Itis, therefore, a principal object lof the invention to v provide an acetalization method which imparts tofpolyvinyl alcohol fibersa sufficientk resistance to boiling water lbutis not accompanied by=a decrease of the affinity.

ofthe fibers tov dyestuffs. r

Other objects" and advantages willbe apparent from`V i.

aconsiderationuof the-specification and claims.

As pointed out hereinabove, vit is necessary 'to ad i alkali met-al or ammonium salts in an amount of at least When the polyvinyl alcohol fiber has vbeen prepared by spinning a polyvinyl alcohol solution into a coagulating .bath containing an inorganic salt such as-'sodium or ammonium sulfate, a certain amount of saidsalt willadhere Ito the fiberv andfwill be Vcarried therewith into the` acetalizing bath. yThis amount `has to` be taken into laccount in adjusting the concentration of the,y acetalzing sufficient to raisethe salt content ofthe acetalizing-bath above 57%V by weight shouldbel'avoided.

Accordingly, it is often necessary to decrease the salt content o f suchnfibers before they .pass into-.the v'acetalization bath. InY general, 'it is 'difficult'to vrenplofve the salt f Whenthe fiberfistwashedbefore Ythe heat-treatment', l itisnecessary'to leave at least5' tov10%. of salt/entireV Vber inr order ,tol insure that?v the filaments not' stick `together 'during drying and do not becomefdiscolored Patented lit/lar. 255, i951 l in the heat-treating step. The heat-treated fiber can be washed to a salt content of less than percent, but it takes a very long time to Wash out the salt completely. When the heat-treated fiber is washed with water to a salt content of less than percent and then squeezed between rolls to a moisture content of less than- 200 percent, it is possible to keep the salt concentration in the acetalizing bath at the desired proportion of less than 5 percent.

The concentration of the inorganic acid in the acetalizing bath has a little influence on the dyeing properties of the acetalized fibers. Higher concentrations, however, may affect the mechanical properties of the fiber. Sulfuric acid, for instance, tends to decrease the tensile strength of the fiber when present in concentrations above percent, and in concentrations of 25 to 30 percent it may solubilize the fiber in the acetalization bath. But we found that an addition of organic acids having not more than 5 carbon atoms considerably increases the affinity of the fibers to dyestuffs without substantially affecting their mechanical properties. Such acids are, for instance, formic acid, acetic acid, propionic acid, butyric acid, monochloroacetic acid, glycolic acid, lactic acid, and others; they may be added in amounts up to 20 percent based on the Weight of the acetalizing bath.

A modification of our new process consists in carrying out the acetalization in two steps. In the first step, the liber is acetalized in a conventional acetalization bath to an acetalization degree of less than 20 percent; the second and final acetalization step is carried out in an acetalizing bath according to our invention, i.e. in a bath containing less than 5 percent of alkali metal or ammonium salts, and, if desired, an organic acid.

The relationship between the acetalizing bath come position, acetalization degree and dyestuff absorption is shown, by way of example, by the graphs in the accompanying drawing, in which y Figs. la-lc show the absorption of dyestuffs as a function of the acetalization degree obtained in solutions containing varying amounts of sulfuric acid, formaldehyde and sodium or ammonium sulfate;

Fig. 2 is a graph showing the effect of additions of inorganic sulfates other than sodium or ammonium sulfate;

Fig. 3 is a graph showing the influence of additions of organic acids;

Fig. 4 isa similar graph showing the influence of various chlorides added to an acetalization bath containing hydrochloric acid; and

Fig. 5 illustrates a two-step acetalization process.

The polyvinyl alcohol fibers used for these tests had been heated according to theV method disclosed in the copending application Serial No. 300,375, recited above, for 5. minutes in super-heated steam of 200 C. with a water vapor pressure .of 2.3 atm. abs. and hadbeen washed with waterat room temperature so as to remove substantially the sodium sulfate adhered to the fibers. The fibers had a softening temperature in water (as defined hereinabove) of 85 C. The different acetaliza-V tion degrees of the fibers were obtained by proper control of the acetalization time. Y

As will be noted on inspection of Fig. 1, the dyestu absorption increases with decreasing sodium or ammonium sulfate content'of the acetalizationbath. For each salt concentration, the dyestuft absorption decreases generally, in the range of higher acetalization degrees, with increasing acetalization. This would suggest a procedure to acetalize the fibers as little as possible. higher acetalization degree is necessary to obtain a high wet-heat resistance of the fibers. This is indicated by the dotted and solid parts of the curves. The dotted sections designate the range where the fibers shrink in water of 100 C. by more than 5 percent, i.e. where they have an insufficient wet-heat resistance. fibers do not shrink more than Spercent at 100 C. is

The range where thel However, a p

i designated by the solid sections, and it will be noted that this range corresponds in almost all cases to a minimum acetalization degree of at least 20 percent, i.e. where at least 20 percent of the original hydroxyl groups of the polyvinyl alcohol are acetalized. On the other hand, too high a degree of acetalization is also objectionable because highly acetalized polyvinyl alcohol has a low heat resistance in air. The acetalization degree should be about 25 to at most 50 percent.

The acetalizing solutions used in the tests of Fig. la contained by weight 5 percent of formaldehyde, 15 percent of sulfuric acid and 0, 5,l 15, and 25 percent of sodium sulfate, or 5 and 15 percent of ammonium sulfate, respectively.

In Fig. 1b, the formaldehyde content was raised to 10 percent, and in Fig. 1c the sulfuric acid content was raised to 20 percent.

T he acetalization was in all cases carried out at 50 C. for times suicient to obtain the indicated acetalization degrees.

The dyestuffabsorption is expressed in mg, of the dyestuff Direct Fast Violet BB conc. (Color Index No.V

325) absorbed within 1 hour at 70 C. by l g. of fiber from 30 cc. of a solution containing 2 percent of the dyestuff, based on the weight of the fiber.

The dyestuft absorption of cotton is about 10 mgJg. fiber and that of viscose rayon about 15 mg./g. fiber. The graph shows that the dyestuff absorption of polyvinyl alcohol fibers is comparable. with-that of cotton and viscose rayon, if the polyvinyl alcohol fiber has been acetalized with acetalizing solutions containing not more than 5 percent of sodium or ammonium sulfate.

Similar figures for the dyestuff absorption are obtained when 'other dyestuts are used, such as sulfide, vat, naphtbol, acid, mordant, basic, and acetate dyestuffs.

The graph of Fig. 2 shows that salts of sulfuric acid other than alkali metal or ammonium sulfate have smaller effects on the dyeing properties of the acetalizedpolyvinyl alcohol fibers; the effect of zinc sulfate is the least noticeable, that of magnesium sulfate is small and that of aluminium sulfate is a little more pronounced. The other sulfates, for example, the sulfates .of cadmium, chrome, iron, copper or manganese Vhave ,little effect, and only potassium sulfate behaves like sodium sulfate.

The graph of Fig. 3 is similar to that of Fig. 1 and illustrates the favorable influence of organic acids on the dyeing properties of the acetalized polyvinyl alcohol ber. It will be noted that fibers acetalized in the presence of acetic and formic acid have an afinity to the dyestut which is greater than that of cotton and about equal to that of Viscose rayon. j v l The graph of Fig. 4 is similar to the preceding figures and illustrates Vthe infiuence of the various chlorides (sodium chloride, ammonium, chloride, and zinc chloride) added to the acetalization bath containing hydrochloric acid on the dyeing properties of the acetalized polyvinyl alcohol fiber.

inthe tests of Fig. 5, the fiber was first acetaliz'ed at 50 C. for 10 minutes Vto an acetalization degreegof 10 percent in a bath containing by weight l5 percent of sulfuric acid, 5 percent of formaldehyde and l5 percent of sodium sulfate. The acetalization was then con tinued -on the one hand in a bath containing 15 percent of'sulfuric acid, 15 percent of formaldehyde, and 15 percent of sodium sulfate, and on the other hand in a bath containing only 15 percent of sulfuric acid and 15 percent of formaldehyde. The curvesdemonstrate clearly the superiority of the after-acetalization with solutionsV containingless than 5 percent of the salt. We haveV for* ther found'that similar results as described hereinbefore are obtained when hydrochloric acid is used instead of sulfuric acid.l

The following illustrative examples.v are given, vwithout in any way limiting the invention, to Vshow methods of acetalizing polyvinyl alcohol fibers without impairing their dyeing properties. In the examples the salt ladhering to the polyvinyl alcohol iibers was not removed except where particularly noted; such an excess of acetalizing bath with respect to the amount of treated fibers was used that the concentration changes. caused by the salt carried by the bers were negligible.

All figures are given by Weight.

Example 1 An aqueous polyvinyl alcohol solution Was spun into a coagulating bath containing sodiumsulfate; the obtained fiber Was stretched during and after spinning, dried and heated in superheated steam'of 220 C. at a vap'or pressure of 2 atm. for a time of5 min. The fiber, which had a softening temperature (in water) of 90 C., was acetalized at 50 C. for one hour insa bath containing 3 percent of formaldehyde, 3 percent of sodium sulfate and 14 percent of sulfuric acid.

The obtained liber had an acetalizationrdegree of 30 percent. in a bath containing 5 percent of Direct Black GX (Color Index No. 581) at 70 C. for 1 hour, a deep black shade was produced.

When the same liber was acetalized under the same conditions in a bath containing percent of sodium sulfate (instead of 3 percent), and dyed in the-same manner, only a grey shade was obtained.

The mechanical properties of the fibers are hardly influenced by the different composition of the acetalizing bath. The tensile strength of the lliber acetalizedfin the bath containing 3 percent of salt was 2 .9 g./d., that of the ber acetalized in the bath containing 15 percent of salt Was 3 g./d.

Example 2 The heat treated liber of Example 1 having a softening temperature of 90 C. was acetalized at 60 C. for one hour in a bath containing 4 percentvof formaldehyde,- 16

percent of sulfuric acid and 5 percent of sodium sulfate.

The obtained fiber, which had an acetalization degree of 37 percent, was dyed at a goods-liquor ratio of 1:100

in a bath containing 4 percent of Direct Brown (Color v ndex No. 420) to a dark brown shade.

. The fiber acetalized at 60, C. for one hour ina bath containing 4 percent of formaldehyde, 19 percent of sulfuric acid, and percent of sodium sulfate had the same acetalization degree but could be dyed under the same conditions only to a light tan shade.

Example 3 When the same fiber-was acetalized in a bathcontaiib ing 8 percent of polyoxymethylene, 15 percent of sulfuric acid and 15 percent of-sodium sulfate to an acetal izationdegree of percent, anddyed as before, only 4.8 mg. of the dyestuflc were absorbed by 1 grof the ber.'

Example .4

A dry spun polyvinyl alcohol fiber. .was heatedvfor 5 min. in air of 215 C. to raise the softening temperature When dyed at a goods-liquor ratio of 1:100.

in Water to 81 C. Theber-Was then; acetalized for 1 hour at 50C. in a bath containing 5 percent of formaldehyde,f2 0 percentof sulfuric `'acid andno salt..l ,The

acetalized fiber, whichhad an acetaliZation-degreeo 40 percent, 'was blendedlwith .viscose rayon andv could be dyed with direct colors to the `same.` `s'hade aslthe'` rayon fiber.

Example 5 A dry spun polyvinyl alcohol liber heated in air of 215 C. and having a softening temperature in water of 81 C. was acetalized for l hour at 50 C. infa bath containing 3 percent of formaldehyde and 3 percent of hydrochloric acid but no salt. The liber had an acetalization degree of 35 percent and showed excellent aiiinity to direct dyestufs. Example 6 A polyvinyl alcohol fiber obtained by wet-spinning an aqueous polyvinyl alcohol solution into a coagulating bath containing ammonium sulfate Was dried and heattreated in superheated steam having a vapor pressure of'S lig/cm.2 at 160 C. for 20 min. The ber was washed to a content of 4 percent of ammonium sulfate by passing it through water and squeezing it between rolls to a moisture content of percent. The fiber was then continuously acetalized by passing vit at 50 C. for `1 hour through a bath containing v2 percent of hexamethylenetetramine and 20 percent of sulfuric acid.A The liber, which on leaving the bath had an acetalization dcgree of 32.0 percent, was squeezed between rolls to a moisture content of 100 percent and could be dyed with Vthe acid dyestui Naphthylamine-Black 10B (Color Index No. 246) to a deep black shade.

lf the same fiber was acetalized to about the same acetalization degree in a bath containing the same .amounts of hexamethylenetetramine and sulfuric acid,

but in. addition l15 .percent of ammoniunr sulfate, the ii-ber could be dyedunder the same conditions only' to a light grey shade.

' Example 7 i AExample- .8r The heat-treated polyvinyl alcohol ber of Example 2 isl passed into an aqueous acetalizing bath having artemperature of'.50 C. and containing 3 percent of'acetaldehyde and 15y percent of sulfuric acid. After half an hour the fber'has an acetalization degree of`4l.0%.

Ifthe samei acetalizing bath is used with an addition `of lOpercent of sodium sulfate, the acetalizationdegree is 37 percent. n

The bervacetalized in 'fthe iirstbath has a higher affinity to direct dyestusthan ,the fiber acetalized in the second bath. .y

Example 9 'A pt )ly'vinyl .alcohol fiber obtainedbyv a -convention-al Wet spinning process is stretched by 2 0 percent o f its original length inv air off 225 C.fd'ur ing 5`rnin. ."The

heat-treated=andstretchedberhas r4a softeningpointlin wat'erofgSZz" `The liber is then immersedat 60 C.'-

forf50'minutes in.Y an aqueous solution cont`ainirig17 5 'ipercent ,of formcA acidyan'dS percent'of'fornialdehj/de.;j i

normasl 'Z This formalized fiber has an acetalization degree of 39 percent.

If the sarne fiber is immersed under the same conditions in the same bath but y,not containing formic acid, the acetalization degree is 35%.'

When both fibers are dyed with 2 percent of Benzobrown MC (Color Index No. 420) for l hour at 60 C., the fiber acetalized in the presence of formic acid obtains a much deeper shade than the fiber acetalized Without formic acid, and the color depth of the former can be compared with the shade obtained in viscose staple fiber.

Example 10 A polyvinyl alcohol fiber obtained by a dry spinning process is heated for 30 minutes in superheated steam of 160 C. with a vaporpressure of 5 kg./cm.2 and then acetalized in an aqueous solution containing 4 percent of hydrochloric acid, l percent of acetic acid, and 4 percent of formaldehyde at 50 C. for 30 min. to an acetalization degree of 38 percent. if acetalized under the same conditions Without acetic acid, t'ne acetalization degree -is 32 percent. On the first fiber, Sulphur Blue R (Color Index 959) produces a much deeper shade under the same dyeing conditions than on the fiber acetalized without acetic acid.

Example 11 A polyvinyl alcohol fiber obtained by a Wet spinning process is heated for 5 minutes in air of 220 C. One portion of the fibers is placed for half an hour at 50 C. in a solution (A) containing 15 percent of sulfuric acid, percent of butyric acid, and 3 percent of acetaldehyde, the other portion under the same conditions in a solution (B) containing only percent of sulfuric acid and 3 percent of acetaldehyde. The degree of acetalization is for fiber (A) 46%, for fiber (B) 44%. When dyed with Chromoxane Pure Blue B (Color index No. 720), fiber (A) is dyed a much deeper shade than fiber (B).

Example 12 The heat-treated ber of Example 10 is immersed for 15 minutes in an aqueous solution of 60 C. containing l5 percent of sulfuric acid, 5 percent of formaldehyde, and 15 percent of sodium sulfate. The fiber has then an acetalization degree of 14.5%. Portions of this slightly acetalized fiber are consequently placed for 50 minutes at 60 C. either in a solution (A) containing 15 percent of sulfuric acid, 5 percent of monochloroacetic acid, 10 percent of methanol and l0 percent of formaldehyde, or in a solution (B) of the same composition without the monochloroacetic acid. The acetalization degree of fiber (A) is 39%, that of fiber (B) 37%. When both fibers are dyed with Celliton Fast BlueV FFR, fiber (A) is dyed a deeper shade than fiber (B).

Example 13 Heat-treated fibers according to Example 10 are placed for 1 hour lat 50 C., either in an aqueous solution (A) containing l5 percent of sulfuric acid, 10 percent of lactic acid, and 5 percent of formaldehyde, or'in a solution (B) containing only 15 percent of sulfuric acid and 5 percent of formaldehyde. When dyed with Indanthrene Blue RSN (Color Index No. 1106), fiber (A) assumes a deeper shade than fiber (B).

Example 14l sodium sulfate and these acetalized fibers are'dyed with Diarnine Green B (Color VIndex No. 593). The most heavy Shade is @binnenin (A), (e) 'and (C) and the medium shade at (D); (E) and (F) are only stained a very light shade.

The term polyvinyl alcohol as used in the specification and claims is understood to mean not only pure polyvinyl alcohol but also its copolymers with other polymerizable compounds, such as ethylene, maleic anhy dride, acrylonitrile, and the like, provided-such copolymers contain at least percent by weight of polyvinyl alcohol. Such copolymers are known in the art and have been proposed for the preparation of textile fibers in the same Way as polyvinyl alcohol.

The'term water-soluble lower aliphatic aldehyde" as used in the claims is understood to cover also compounds of such aldehydes specified above, as polymers and amination products, which split off the aldehyde in the acid solution.

What we claim is:

1. A process for improving the dyeing properties of polyvinyl alcohol fibers, the shrinkage of which has been reduced by a heat treatment to a shrinkage of not more than 5 percent of their length in Water of 80 C., said process comprising treating said fibers with an aqueous solution containing by Weight about .5 to 15 percent of formaldehyde, about .5 to 25 percent of an inorganic acid selected from the group consisting of hydrochloric acid and sulfuric acid, and an inorganic salt selected from the group consisting of ammonium and alkali metal salts of said acid in a concentration not exceeding 5 percent of said solution, for a, timev and lat a temperature sufiicient to acetalize about 25 to 50 percent of the hydroxyl groups of the polyvinyl alcohol.

2. A process for improving the dyeing properties of polyvinyl alcohol fibers, the shrinkage of which has been reduced by a heat treatment to a shrinkage of not more than 5 percent of their length in water of 80 C., said process comprising treating said bers with an aqueous solution containing by weight about .5 to 15 percent of formaldehyde, about .5 to 25 percent of an inorganic acid selected from the group consisting of hydrochloric acid and sulfuric acid, an inorganic salt selected from the group consisting of ammonium and alkali metal salts of said acid in a concentration not exceeding 5 percent of said solution and about .5 to 25 percent of an alkyl carboxylic acid containing not more than 5 C atoms, for

a time and at a temperature sufficient to acetalize about t 25 to 50 percent of the hydroxyl groups of the polyvinyl alcohol. y

3. A process for improving the dyeing properties of polyvinyl alcohol fibers comprising spinning an aqueous solution containing polyvinyl alcohol into a coagulating bath containing a coagulating salt selected from the group consisting of alkali metal and ammonium sulfates and chlorides, heating the obtained filaments to a temperature of about to 225 C. until said filaments shrink less than 5 percent of their length in water of 80 C.,re moving the coagulating salt adhered to the filaments from the coagulating bath to such an extent that the filaments contain not more than 10 percent by Weight of said salt, and treating said filaments with'an aqueous acetalizing solution containing by weight about .5 to l5 percent of formaldehyde, about .5 to 25 percent of the acid of said coagulating salt, and said coagulating salt inV such an amount that saidV salt total concentration in the solution does not exceed 5 percent, for a time and at` a temperature sufficient to acetalize about 25 to 50 percent of the hydroxyl groups of the polyvinyl alcohol.

4, The process as defined in claim 3, wherein said aqueous acetalizing solution contains,'in addition about .5 to 25 percent of an alkyl carboxylic acid containing not more than 5 carbon atoms.

5. A process forV improving the; dyeing'properties'of polyvinyl alcohol fibers, the ,shrinkage of which haspbeen reducedvby a heat treatment rto ashrinkageof not more lthan 5 percent' of( their length-in water of 80 C., said process comprising first acetalizing not more than 20 percent of the hydroxyl groups of the polyvinyl alcohol with formaldehyde and then treating said acetalized bers with an aqueous acetalizing solution containing by Weight about .5 to 15 percent of formaldehyde, about .5 to 25 percent of an inorganic acid selected from the group consisting of hydrochloric acid and sulfuric acid, and an inorganic salt selected from the group consisting of arnmonium and alkali metal salts of said acid in a concentration not exceeding 5 percent of said solution for a time and at a temperature suicient to raise the total acetalization degree of the polyvinyl alcohol to about 25 to 50 percent. f

10 6. The process as dened in claim 5, wherein said aqueous acetalizing solution contains, in addition, about .5 to 25 percent of an alkyl carboxylic acid containing not more than 5 carbon atoms.

References Cited in the tile of this patent UNITED STATES PATENTS 2,360,477 Dahle Oct. 17, 1944 2,393,595 Dawson Jan. 29, 1946 2,591,242 Drisch Apr. 1, 1952 2,636,803 Cline et al. Apr. 28, 1953 2,636,804 McCellan et al. Apr. 28, 1953 2,639,970 Tomonari May 26, 1953 

3. A PROCESS FOR IMPROVING THE DYEING PROPERTIES OF POLYVINYL ALCOHOL FIBERS COMPRISING SPINNING AN AQUEOUS SOLUTION CONTAINING POLYVINYL ALCOHOL INTO A COAGULATING BATH CONTAINING A COAGULATING SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AND AMMONIUM SULFATES AND CHLORIDES, HEATING THE OBTAINED FILAMENTS TO A TEMPERATURE OF ABOUT 140 TO 225*C. UNTIL SAID FILAMENTS SHRINK LESS THAN 5 PERCENT OF THEIR LENGTH IN WATER OF 80*C., REMOVING THE COAGULATING SALT ADHERED TO THE FILAMENTS FROM THE COAGULATING BATH TO SUCH AN EXTENT THAT THE FILAMENTS CONTAIN NOT MORE THAN 10 PERCENT BY WEIGHT OF SAID SALT, AND TREATING SAID FILAMENTS WITH AN AQUEOUS ACETALIZING SOLUTION CONTAINING BY WEIGHT ABOUT .5 TO 15 PERCENT OF FORMALDEHYDE, ABOUT .5 TO 25 PERCENT OF THE ACID OF SAID COAGULATING SALT, AND SAID COAGULATING SALT IN SUCH AN AMOUNT THAT SAID SALT TOTAL CONCENTRATION IN THE SOLUTION DOES NOT EXCEED 5 PERCENT, FOR A TIME AND AT A TEMPERATURE SUFFICIENT TO ACETALIZE ABOUT 25 TO 50 PERCENT OF THE HYDROXYL GROUPS OF THE POLYVINYL ALCOHOL. 